LEDs are typically formed as dies having an anode terminal and a cathode terminal. An LED die is typically mounted on a larger substrate for heat dissipation and packaging. The substrate may contain additional circuitry, such as a passive electrostatic discharge device. The LED die and optional substrate are then typically packaged, where the package has robust anode and cathode leads for being soldered to a printed circuit board (PCB).
LEDs may be controlled by a current source to achieve a desired brightness. The current source may be a MOSFET or a bipolar transistor formed in a separate die. The current source and LED are typically connected together by wires or a PCB.
Providing the current source separate from the LED die requires extra space and interconnections, adding cost. Other disadvantages exist, including the possibility of mismatching components. It would be desirable to provide a very compact LED module with an integrated current source driver circuit.
Additional problems arise when driving multi-colored LEDs, such as in a color display or for creating a white light source. An LED is a two terminal electrical device with non-linear voltage versus current characteristics. Below a particular voltage threshold, the LED is high impedance. Above the threshold, the LED's impedance is much lower. This threshold depends primarily on the bandgap of the semiconductor LED. The bandgap is selected for a particular peak emission wavelength. Red LEDs have bandgaps on the order of 2 eV, blue LEDs have bandgaps on the order of 3 eV, and green LEDs have bandgaps between 2 eV-3 eV. Since the forward voltage is directly related to the bandgap energy, red, green, and blue LEDs cannot simply be connected in parallel to output a desired color or light; each color LED must have its own driver circuit. The different materials (e.g., GaAs, GaN, etc.) used to form the different color LEDs also affect the forward voltages. Further, even within LEDs outputting the same wavelength, their forward voltages vary due to process variations, so even connecting the same color LEDs in parallel is problematic. Providing a separate driver circuit for each LED and interconnecting it to the LED adds space and cost. This is particularly problematic when trying to minimize the size of an RGB pixel in a display.
LEDs can be organized in passive matrix addressable arrays. For instance, a set of LEDs can be connected with their cathodes connected to a row select driver and their anodes connected to a column data bus. Several of these rows can be used to form a larger array addressable by row and column. Providing a controlled current through an addressed row-column will energize the LED(s) at the addressed location(s) to emit the desired color and intensity of light, such as for a color pixel in a display. Since the interconnection between the LEDs is non-zero impedance, the voltage drop throughout the interconnect network can inadvertently forward bias a non-addressed set of LEDs. Such incidental forward bias will cause excess light in non-addressed segments, which reduces light-to-dark contrast of the array.
This problem is made worse by placement of downward facing printed LEDs, where the downward facing LEDs are for reverse bias transient voltage suppression. The downward LEDs are anti-parallel to the upward facing LEDs. In a simple addressing scheme, only the upward LEDs are intended to emit light. When a row is not selected, the associated LEDs are biased at a sub-threshold voltage or possibly reversed biased. Anti-parallel downward LEDs are problematic if the un-selected rows are reversed biased, which forward biases the downward LEDs causing them to emit light, reducing the light-to-dark contrast of the array.
It would be desirable to create integrated LED modules that avoid the above-mentioned problems when connected in an addressable array.
It would also be desirable to create integrated LED modules where LEDs of different colors can be connected in parallel to form a high density of compact RGB pixels.
It would also be desirable to create integrated LED modules of different colors that can be inexpensively packaged together in a single panel for generating light for backlighting, for general illumination, or for a color display.
It would also be desirable to create an interconnection and addressing scheme for multiple LED modules to form a compact light or display panel.